Locator device for continuously locating a dipole magnetic field transmitter and its method of operation

ABSTRACT

A locator/monitor for locating a boring tool and monitoring the progress of the tool for control purposes. The locator/monitor is used in expedited locating methodology and straightforward calibration techniques of the present invention. A durable and cost effective pitch sensor is also provided by the present invention. In addition, the present invention provides a slotted transmitter housing formed of an electrically conductive material, where the magnetic field generated by the transmitter is capable of penetrating to the surface.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Pat. application No.662,939, filed Mar. 1, 1991, now U.S. Pat. No. 5,155,442.

TECHNICAL FIELD

The present invention relates to apparatus capable of locating and/ormonitoring the position (i.e., the depth below a surface and thelocation within the horizontal plane at that depth) and/or orientation(i.e., yaw, pitch, roll or a combination thereof) of a device locatedout of view below a surface. More specifically, the present invention isdirected to locator/monitor devices that are suitable for use incombination with boring apparatus.

BACKGROUND OF THE INVENTION

Utilities are often supplied from underground lines. Two techniques aregenerally used to install such lines. In one technique, the utility linepathway is excavated; the line is installed; and the excavated materialis replaced. While this method is suitable for new developments,implementation of this technique is not always practical in previouslydeveloped areas. As a result, industry development efforts have beenfocused on excavating tools capable of installing utilities undergroundwithout surface disruption.

Several guided and unguided boring tools are currently on the market.Guided tools require substantially continuous location and orientationmonitoring to provide the necessary steering information. A prerequisiteof such monitoring is, of course, locating the tool that is to bemonitored. Only once the position of the tool is located can a properdepth measurement be obtained, for example, from a measuring positiondirectly above the head of the boring tool which houses a transmitter.Unguided tools would also benefit from periodic locating orsubstantially continuous monitoring, for example, in prevention ofsignificant deviation from planned tool pathways and close toolapproaches to utilities or other below surface obstructions.

Locating or monitoring systems currently used in combination with boringapparatus are either cable locating systems or are based on cablelocating technology. Although the more advanced systems performadequately, limitations on cable locating technology also limitmeasurement accuracy.

Most cable locators involve receiver detection of an oscillatingmagnetic field derived from electrical current directly fed or inducedonto the cable. The magnetic field lines emanating from a cable areessentially cylindrical in shape, forming concentric circles around thecable. As the current flows along the cable, losses occur as a result ofdisplacement and induced currents into the soil. Consequently, the exactsignal strength of the magnetic field emanating from the cable at anypoint is unknown. Although local signal peaks or nulls (depending onreceiver antennae and electronic configuration) are useful to determinethe surface position directly above the cable, signal strength (i.e.,magnetic field strength) alone is not directly indicative of cabledepth. In certain specific circumstances (i.e., when the rate of lossalong the cable length is not great), a signal strength ratio can beused to compute depth. If the cable run is straight for a long distance(compared to the depth), the magnetic field strength (B) will beinversely proportional to the distance (d) from the cable to thereceiver (i.e., B α1/d or B=k/d, where k is a proportionality constant).By taking two signal strength readings at different locations directlyabove the cable, the proportionality constant can be eliminated and thedepth determined.

A simple device for determining the depth of a relatively straight cableis manufactured by Dynatel, a subsidiary of the Minnesota Mining andManufacturing Company. The Dynatel device includes a single antenna, again control knob and a gain doubling switch. The operator determinescable depth by (1) placing the device on the ground above the cable; and(2) adjusting the output displayed on a meter with the gain control knobuntil the meter needle lines up with a line on the meter scale; (3)doubling the gain with the switch therefor; and (4) vertically elevatingthe device until the output returns to the original value (i.e., theneedle realigns with the meter line referred to in step (2)). Since themagnetic field strength is inversely proportional to the distance, theheight of the unit above the ground at step (4) is equal to the depth ofthe cable. This procedure is accurate, but time consuming. It alsobecomes impractical for more deeply buried cables, requiring theoperator to raise the device above his head.

Other currently used cable locating devices employ two antennae andlogic circuitry to determine depth. The antennae are separated by afixed distance. With this known separation distance and magnetic fieldstrength readings at the antennae, cable depth can be computed. Thedifficulty with these devices is that there are practical limitsregarding antennae separation. If the cable depth is much larger thanthe antennae separation, which is generally approximately 12 to 18inches, signal strength measurement accuracy becomes more critical.Measurement accuracy is affected by differential drifting of theelectronics associated with the antennae as well as differentialresponses of the antennae themselves.

Various approaches have been taken to enhance magnetic field strengthmeasurement precision. The accuracy of these approaches increases as thenumber of components common to the two measurement circuits increases.Current systems accomplish this by taking a magnetic field reading atone antenna; switching the electronics connection from one antenna tothe other; and measuring the magnetic field strength at the secondantenna. Although this switching methodology eliminates many sources oferror, one major error source remains--the antennae. To increasesensitivity, ferrite rods are sometimes employed to enhance theeffective capture area of the antennae. As a result of the antennaeseparation, both antennae may not experience the same thermalenvironment. The characteristics of ferrite vary measurably withtemperature and are not consistent between rods. Alternatively, largediameter air-core coils are employed. Such coils eliminate theinconsistency of the ferrite rods, but still exhibit thermal driftproblems. Air-core coils also are generally larger in diameter.

All of these spatially separated two-antenna devices must beperiodically calibrated. Any aging or drifting of an antennae will causerapid loss in cable depth measurement accuracy, particularly at depthsthat are large compared to antennae separation. In cable locators, thisis generally not a serious problem, since most cables are buried atdepths of less than 2 or 3 times the separation.

A device conforming to the above-described arrangement is available fromRadiodetection Ltd. (Bristol, England), the RD300. The device includestwo antennae with horizontal coil axes disposed a fixed verticaldistance from each other. In operation, the device is placed on theground, such that a first receiving antennae sensor is near ground level(e.g., within about 1-2 inches) and a second receiving antennae islocated about 16 inches thereabove. The ground therefore serves as areference surface for depth measurement. One disadvantage of thisparticular prior art device and other devices that operate similarlythereto manifests itself when the reference surface exhibits anobstruction such as a curb, a rock, landscaping or the like, at adesired measurement location. Under these circumstances, an operatormust compensate for the obstruction to obtain the depth below thereference surface. Another disadvantage of this equipment is that thedepth measurement process is time consuming even after the device isproperly located above the transmitter (i.e., a needle must be alignedwith a meter line through a knob-actuated adjustment process).Radiodetection Ltd. applies this technology to cable, sewer and pipelocation as well as horizontal boring tool monitoring.

The principal means of locating a boring tool head for guidance purposesis to place a radio frequency transmitter in the tool head, and trackthe tool from the surface using a radio frequency receiver that detectsthe alternating magnetic field emanating from the transmitter. Whilethis is similar to the cable-locating situation, the type of measurementnecessary for accurate guided boring differs, and the requirements therefor are more stringent. Transmitters or sondes generally emit a dipolemagnetic field in the normal measurement range, which differs from thesource or source-like magnetic field emanating from a utility cable.When a single horizontal antennae is used to measure the strength of adipole magnetic field, that parameter varies as depicted in FIG. 1a.

A transmitter 10 is located directly below a maximum field strengthpoint 12. Nulls 14 are present in the horizontal field directly aheadand behind maximum 12, causing local peaks 16 in field strength. If alocator/monitor operator were to commence operations at a locationsubstantially ahead or behind the actual transmitter 10 location, hemight locate one of local peaks 16 and believe the tool to be directlybelow. In order to be certain that field strength maximum 12 has beenlocated when using single horizontal antennae devices, another peak mustbe found and evaluated to be lower in strength (i.e., to be a local peak16). An operator failing to take this precautionary measure may concludethat transmitter 10 is located at a position that leads or trails itstrue location. Erroneous depth readings and subsequent misplacement ofthe bore typically result.

A single vertical antennae fares no better. A vertical antennae willproduce a null directly above the transmitter. This null exists along aline extending on both sides of the transmitter, however, and thereforecannot be used to locate a point, such as the transmitter location. Datafrom a combination of two antennae may be manipulated to provide a moreaccurate indication of transmitter location. An orthogonal set ofantennae can produce the monotonic signal strength variation shown inFIG. 1b.

When guiding a boring tool, the operator constantly requires accuratedepth measurements, and time consuming procedures, such as the singleantenna cable locator utilizing gain doubling, are therefore notpractical. For tool control purposes, the operator must be able todetermine the depth gradient to ascertain the direction (i.e., up ordown) in which to steer. Gradient determinations require greaterprecision than depth measurement. Also, boring depth may be a factor of0 or more greater than practical antennae separation limits of spatiallyseparated two antennae locators.

U.S. Pat. No. 4,806,869 issued to Chau et al. discusses a 5-sensorreceiver apparatus capable of "locating the position of a boring devicewithin the ground with respect to a particular reference location alongan above ground path directly over the intended course" of the boringdevice. In this receiver, four sensors are arrayed at the four cornersof a square within a horizontal plane (i.e., parallel to the surface),the midpoint of which is displaced vertically from the fifth sensor.Chau et al. indicate that such a receiver is an improvement over a4-sensor device designed to locate/monitor electronically conductivecable, having sensors located at the end points of two intersectinglines of equal length within a plane that is perpendicular to thesurface.

The 4-sensor cable-locating apparatus was not designed for continuousmonitoring. Signals from the horizontally placed sensors are used tolocate the transmitter, while signals from the two vertically alignedsensors are used to determine cable depth. Such a process is impracticalfor continuous monitoring.

In contrast, the 5-sensor apparatus utilizes signals from the twohorizontally disposed sensors, located in the plane perpendicular to thedesired path of the boring device and within which the boring device isactually positioned, and the vertically displaced sensor to determineboring device depth and displacement from its intended path.

The disadvantage of the 5-sensor device is its complexity. This deviceis also susceptible to locating local peaks 16 in the signal strength.Also, the operator of a 5-sensor device traverses the desired boringdevice path, rather than locating a position directly above the device.

Again, these 4- and 5-sensor prior art receivers incorporate sensorsthat are in fixed spatial positions with respect to each other. Incontrast, U.S. Pat. No. 4,646,277 issued to Bridges et al. includes asensing assembly formed of three orthogonal pick up coils. The sensingassembly of the Bridge et al. patent serves as a homing beacon for aboring apparatus, rather than a means to establish the position of thetool head.

U.S. Pat. No. 3,906,504 issued to Guster et al. describes a method oflocating and plotting tunnels using a portable receiver to monitor atransmitter moving through the tunnels. Guster et al. employ an antennaehaving a vertical axis in the transmitter. While this antennaeconfiguration eliminates nulls, such an arrangement is not practical ina boring application, because the head of the boring apparatus rotates.Signal strength emanating from a vertically oriented antennae wouldtherefore vary during boring.

Also, Guster et al. employ very complex mathematics in determining thedistance between the transmitter and the receiver. The need for acalibration system involving complicated electronics for use with theGuster et al. system is discussed, without further explanation, atColumn 2 of the patent. The Guster et al. estimate regarding thecomplexity of calibration electronics appears to be accurate in view ofthe nature of the depth determination employed in the patent.

In addition, Guster et al. employ a pulsed transmitted signal, so as toavoid interference with verbal communication between the receiveroperator and the transmitter operator. Pulsed transmitted signalscomplicate the locating/monitoring process carried out by the receiver.

Steering a boring device also requires information concerning pitch(i.e., angle above or below the X-axis in an XY plane, where the X-axiscorresponds to the longitudinal axis of the boring device and the Y-axisis parallel to the gravity vector). Several pitch sensors are known andcommercially available. Most of these pitch sensors will not produce apitch angle independent of the roll orientation (about the X-axis).Those that can produce a roll-insensitive signal are generally expensiveto produce and easily damaged by shock loads. Less expensivepitch-sensing devices are generally not sufficiently sensitive or welldamped. Because equipment loss is common, most users are reluctant toinvest a large amount of money in components that are deployedunderground. Consequently, development of low cost pitch sensors capableof surviving the loads and environment associated with boring throughsoil, rock and debris has been pursued.

U.S. Pat. No. 4,674,579 issued to Geller et al. describes twopitch-sensing devices. One apparatus features a transmitter thatincludes a mercury switch connected in such a manner that thetransmitter is deactivated when the tip of the housing therefor isupwardly inclined. The inclination of the tip may be determined by anoperator by measuring the angle of rotation at which the transmitterswitches on and off. This type of pitch-sensing device is not highlyaccurate as a result of inaccuracy in measuring the roll angle of thetool head. This process is also time consuming, thereby reducing thepracticality of implementing such a methodology.

The second pitch-sensing device shown in FIG. 8 of and described in theGeller et al. patent includes a first common electrode and twopad-electrode assemblies, including the second and third electrodes,housed within a glass envelope. The glass tube is partially filled withan electrolytic fluid, such that the resistance between the second andthird electrodes and the first common electrode varies with theinclination (i.e., pitch) of the device. This pitch-sensing device canbe costly to implement.

An additional difficulty with locating and monitoring boring apparatushaving a transmitter housed in the boring tool head is that thestructural loads and wear experienced by the tool head require that thehead be fabricated from a high strength material such as steel or someother metal. Since metals conduct electricity, a transmitter containedwithin a metal tool head induces a current in the metal. This inducedcurrent, in turn, induces a magnetic field that cancels the transmittedfield to some extent and, in some circumstances, entirely.

In order to allow the signal emitted by the transmitter to radiate tothe surface, one or more windows or openings have been fabricated ormachined into the conductive boring tool head. Employing this solutionstructurally weakens the tool head and may allow debris or ground waterto enter the tool head and impinge upon the transmitter, therebydestroying the antennae and/or the related electronics. To avoid suchdebris and water damage and in an effort to bolster the strength of thewindowed tool head, these openings have been filled with composite,ceramic or plastic materials, thereby sealing the transmitter andantennae. These filler materials are not as durable as metal, however,and generally fail long before a metal structure would fail. Typically,filler material failure results in costly electronics destruction. Sincethe tool structure is weakened by the window, premature tool headfailures resulting in the loss of both the tool head and the electronicsmay also occur, however.

Another difficulty with the use of the window concept is that theradiated field strength becomes a function of tool head orientation.Specifically, in a single window configuration, the field is strongestwhen emanating from the window and measurably weaker 180° therefrom.Although this result can be useful in determining the tool head rollorientation, it makes it impossible to determine tool depth accuratelywhile drilling, because the tool head is rotating during drilling. Toovercome this restriction, multiple small window or slot tool headdesigns have also been used with mixed success.

In another attempt to overcome this radiated signal problem, the entiretool head structure has been formed with non-conductive materials suchas composites and ceramics. Unfortunately, none of these substitutematerials exhibits all of the desirable characteristics of steel orother durable conductive metals. Strong ceramics do not handle impactloads as well, while composites do not take abrasive wear as well. Thesesubstitute materials are also much more costly than metals.

SUMMARY OF THE INVENTION

The present invention provides a locator/monitor capable of locating aboring tool head for control purposes. The locator/monitor of thepresent invention is compact, portable, easy to carry and user friendly.Accurate boring tool head depth and orientation measurements may beobtained through flexible procedures that may be modified in accordancewith the circumstances under which a measurement is to be made. Preciseand continuous depth and periodically updated orientation measurementsprovide the information necessary to locate and steer the tool head.Depth (i.e., the distance between the reference surface and thetransmitter) may also be presented to an operator of the locator/monitorof the present invention as range (i.e., a monotonic function indicativeof the distance between the receiver and the transmitter).Straightforward calibration and expedited locating methodology may alsobe implemented using apparatus of the present invention.

The locator/monitor of the present invention achieves these goalsthrough the operation of an antennae assembly featuring two orthogonalantennae. The antennae are located in spatial proximity to each other(i.e., they are not disposed a fixed distance apart), thereby decreasingthe size of the locator/monitor and providing monotonic magnetic fieldstrength information. Once calibrated (i.e., the value of theproportionality constant k relating magnetic field strength and range isknown), continuous measurement of range or a gradient thereof, andperiodic updated indications of orientation are possible. Nomanipulation of equipment controls is necessary to initiate or continuegenerating such data.

Calibration of the locator/monitor of the present invention isachievable through a simple procedure. An operator need only locate thetransmitter; deploy the receiver of the locator/monitor of the presentinvention at a first convenient height above the transmitter location;measure the magnetic field strength emanating from the transmitter;deploy the receiver at a second convenient height; and measure themagnetic field strength emanating from the transmitter. To permit thevalue of the proportionality constant to be determined, an independentindicator of the distance between magnetic field strength measurementpoints is provided by locator/monitors of the present invention. Apreferred independent indicator is an ultrasonic receiver-to surfacemeasurement system.

The transmitter may be located in an expeditious manner by "following"dipole magnetic flux lines to the transmitter (i.e., determining theminimum distance to the transmitter, indicated by a maximum magneticfield strength reading as the receiver is rotated) in a stepwisefashion. Staged progress is achieved, because dipole magnetic flux linesare not typically straight line paths to the transmitter (they are localtangents to the flux line along the transmitter axis). Sensitivity ofthis locating procedure can be enhanced by using the square of themagnetic field strength. Ease of accomplishing the locating method isincreased by a beeper or visual function designed to indicate passagethrough a measurement maximum or to predict such passage. The maximumvalue may be stored in memory to permit later comparisons with newmeasurements, with beeper or visual indications occurring when ameasurement equals or exceeds that held value. The signal squaredprocedure may also be used to determine yaw orientation of thetransmitter.

The present invention also provides a pitch sensor capable of supplyingorientation data for devices such as boring tools. The pitch sensor ofthe present invention may also act as a level reference or anaccelerometer. The principal advantages of pitch sensors of the presentinvention are durability and cost effectiveness.

The pitch sensor of the present invention includes a conductive centralrod, running the length of a conductive assembly; two sections ofconductive tubing separated by a small gap, where the length of tubesections and gap correspond collectively to the length of the conductiveassembly; and electrically conductive fluid disposed within theconductive assembly in an appropriate amount. The amount of conductivefluid is selected, such that the central rod of the pitch sensor iscontacted by the conductive fluid when the pitch sensor is in ahorizontal position.

The present invention also provides a transmitter housing formed of aconductive material such as a metal, where the magnetic field generatedby the transmitter is capable of penetrating to the surface. In thismanner, the structural strength of the housing is preserved; theelectronics are protected from debris and water infiltration; and asymmetrical magnetic field is produced by the transmitter.

Radiated signal strength is enhanced by increasing the equivalentinduced electrical current path length in the conductive metaltransmitter housing of the present invention. This increase is achievedby the presence of slots in the housing structure. Preferably, thediameter of the antennae coil is small in comparison with the housingdiameter. In addition, an increased number of slots consistent withmaintaining the structural integrity of the housing is also preferred.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a indicates magnetic field strength as measured by a singleantenna.

FIG. 1b indicates magnetic field strength, as measured by two orthogonalantennae.

FIG. 2 shows a partial cross-sectional view of a typical horizontalboring operation.

FIGS. 3a and 3b show block diagrams of a transmitter of thelocator/monitor of the present invention.

FIG. 4 shows a cross-sectional view of a boring tool incorporating atransmitter of the locator/monitor of the present invention.

FIGS. 5a and 5b show block diagrams of a receiver of the locator/monitorof the present invention.

FIG. 6 shows a cross-sectional view of a pitch sensor of the presentinvention.

FIG. 7 shows an electronic circuit that is capable of driving a pitchsensor of the present invention.

FIG. 8 shows typical pitch response curves that a pitch sensor of thepresent invention may be designed to emulate.

FIG. 9 shows a perspective view of a conductive transmitter housing witha magnetic field transmitting antennae disposed therein.

FIG. 10 shows a perspective view of a conductive transmitter housing ofthe present invention with a magnetic field transmitting antennaedisposed therein.

FIG. 11 shows a perspective view of an embodiment of a receiver of thelocator/monitor of the present invention.

FIG. 12 shows a cross-sectional view of a receiver of an embodiment ofthe locator/monitor of the present invention shown in FIG. 11.

DESCRIPTION OF PREFERRED EMBODIMENTS

While the following preferred aspects of the present invention aredescribed with reference to use thereof in combination with boringapparatus operating in a generally horizontal plane, these aspects areamenable to other uses and applications, as will be recognized bypractitioners in the relevant arts. For example, the apparatus of thepresent invention may be designed to determine the magnetic fieldstrength-depth relationship based on an inverse proportionality (.i.e.,magnetic field strength α1/depth) for cable locating purposes.

Boring apparatus that may be used in combination with thelocator/monitor of the present invention are any apparatus capable of ormodifiable to be capable of generally horizontal boring and housing atransmitter in a manner allowing a signal emanating from the transmitterto penetrate sufficiently for surface signal detection. Such boringapparatus are known and commercially available. Exemplary boringapparatus useful with the present invention include Ditch Witch P40 andP80, Tru-Trac, and Jet-Trac (Charles Machine Works), Direct Line(Straight Line Manufacturing) or GuideDrill (Utilx, Kent, Wash.).

A typical horizontal boring operation is shown in FIG. 2. The operationgenerally requires two or more operators. A first operator 20, who maybe located in the vicinity of a starting pit 22, is responsible foroperation of a boring machine 24. A second locator/monitor operator 26is responsible for locating a boring tool head 28 and determiningsteering commands therefor. Tool head 28 is guided around an obstacle 30at a generally constant depth beneath a reference surface 32 until itreaches a termination pit 34. Locator/monitor operator 26 holds areceiver 36 and uses it to locate the surface position directly abovetool head 28. Once locator/monitor operator finds this position,receiver 36 is used to determine the depth of tool head 28. Using alocator/monitor of the present invention, operator 26 can also determinethe orientation (yaw, pitch and roll) of tool head 28.

As stated above, the boring apparatus houses the transmitter componentof an embodiment of the locator/monitor of the present invention thatincludes a receiver and a transmitter. Alternatively, thelocator/monitor of the present invention may consist of a receivercomponent designed to cooperate with a transmitter that is already inplace or has been obtained separately. In either case, transmittersuseful in the present invention are known and commercially available.Exemplary transmitters are 10/SC 0412-8 and 10/SC 0322-8 (RadiodetectionLimited, Bristol, England) and Flocator (Utilx, Kent, Wash.). Apreferred transmitter of the present invention includes a pitch and rollsensor, as described herein. Such preferred transmitters may be usedwith other receivers or as accelerometers or level references in relatedor unrelated applications.

Transmitted dipole magnetic fields are preferred for use in the practiceof the present invention, because, in part, such fields are fairlyconstant with time. For the low transmission frequencies used in boringapplications, ground attenuation is generally not significant. Incontrast to the horizontal cable situation, the magnetic fieldstrength-distance relationship for a dipole magnetic field-generating or-approximating transmitter is inversely cubic along a straight line fromthe dipole. Specifically, the magnetic field strength (B) at a distance(d) from the transmitter may be represented as follows:

    Bα1/d.sup.3

or

    B=k/d.sup.3

where k is a proportionality constant related to the transmitter signalstrength. Because of the inverse cube-relationship between theparameters, the strength of a dipole magnetic field is a very sensitiveindicator of transmitter depth changes. As set forth above, aproportionality constant based on an inverse relationship mayalternatively be determined for cable locating or other appropriateapplications.

FIG. 3a shows a block diagram for a preferred embodiment of transmitter10 useful in the practice of the present invention. Transmitter 10incorporates a low frequency oscillator 40 operating from about 4 kHz toabout 100 kHz, with about 33 kHz preferred. Oscillator 40 drives anamplifier 42 that is amplitude modulated by a modulator 44. Themodulated output of amplifier 42 drives an antenna 46. Modulator 44provides amplifier 42 with a series of digitally encoded signals derivedfrom a pitch sensor 48 and a roll sensor 50. Specifically, digitaloutput signals from sensors 48 and 50 are multiplexed by a multiplexer52 which, in turn, drives a Dual-Tone MultiFrequency (DTMF) generator54. The tone pair produced by DTMF generator 54 modulates the outputsignal of modulator 44. Specifically, the output of modulator 44includes the carrier and two side tones. A tone pair is preferred over asingle tone as input to modulator 44, because the dual tone requirementlessens the probability that a random signal could be interpreted asdata by the receiver. In this system, two legitimate tones are requiredto constitute data. An analogous system is used in touch-tone telephonesto eliminate noise. While this invention will be described withreference to a DTMF generator, it should be understood that othertechniques, such as a pulsed signal on a separate carrier frequency canbe used to advantage in this context.

A block diagram of an alternative and preferred electronicsconfiguration of transmitter 10 of the present invention is shown inFIG. 3b. Transmitter 10 consists of a pitch sensor 48 to measure theattitude of tool head 28 relative to gravity. Pitch sensor 48 providesan analog signal through line 60 to an A/D converter 62. The digitizedoutput of A/D converter 62 is fed by a data bus 64 to multiplexer 52.

Roll or tool head 28 angle is also measured relative to gravity by meansof roll sensor 50, a 12-position mercury switch. The output signal ofroll sensor 50 is in digital format, so it can be directly fed tomultiplexer 52 by a bus 66. Multiplexer 52 is switched between buses 64and 66 by a timer 68. The output signal of timer 68 is dependent uponoscillator 40 frequency fed to timer 68 by a line 70 (i.e., oscillator40 frequency is divided to a much lower frequency by timer 68).

Multiplexer 52 provides four, 4-bit nibbles of data through a bus 72 toDTMF generator 54 which produces tone pairs from a selection offrequencies that differ from the carrier frequency. For example, tonepairs may be chosen from 8 frequencies below 1 kHz. As a result, thetransmitter of the present invention may employ the same or similar DTMFchips as are used in touch-tone telephone applications, with the chipbeing clocked at a slower frequency by timer 68 than would be the casein a telephone application. The tone pair is fed to an amplitudemodulation amplifier 74 through a line 76. The output signal frommodulation amplifier 74 controls the voltage of an output amplifier 78and is fed to output amplifier 78 through a line 80. Output amplifier 78is driven, for example, in class D operation (i.e., output amplifier 78is turned on and off at the carrier frequency, thereby decreasing powerdissipation) at, for example, 32768 Hz by oscillator 40. The amplitudemodulated signal is fed to a capacitor-inductor pair (82, 84) operatingat series resonance of, for example, 32768 Hz. Inductor 89 is preferablyan antennae producing a dipole magnetic field. Oscillator 40 frequencymay be any frequency that does not interfere with the DTMF generator 54tone pair and that is not subject to substantial ground attenuation.

Regulated 5 volt power is provided to transmitter 10 by avoltage-controlled switching regulator 86 to which current is suppliedby a battery 88. The individual components of transmitter 10 are knownand commercially available, with the exception of the preferred pitchsensor described below. As a result, one of ordinary skill in the artcould construct and implement transmitter 10, as contemplated by thepresent invention.

A cross-sectional view of transmitter 10 housed within a typicaldirectional drilling tool head 28 is shown in FIG. 4. Transmitter 10 issized and configured for sliding engagement within an elongated opening100 formed in a steel tool body 102. A keyed end section 104 oftransmitter 10 consists of a semicircular element 106 which engages asimilar keyed head element 108 located on tool head 28. Battery 88 isalso slidingly engaged within opening 100 and contacts transmitter 10,thereby making one electrical connection. The other electricalconnection is made through a spring 110 attached to a remaining drillstring 112 through a set of threads 114. One or more elongated slots 116in steel tool body 102 provide for penetration of steel tool body 102 bythe magnetic field generated by transmitter 10.

While the interface between transmitter 10 and tool body 102 isdescribed and depicted as a keyway/key arrangement, any otherinterfacing mechanism capable of stabilizing transmitter 10 within toolbody 102 at an appropriate orientation may be used. The appropriateorientation of transmitter 10 is any one in which data from pitch sensor48 and roll sensor 50 may be properly related to the pitch and roll oftool head 28. Similarly, a specific set of electrical connections andstructure for making them are described and depicted. Any similarmechanism may be employed to achieve this end. One of ordinary skill inthe art could therefore interface transmitter 10 with boring apparatus24 and provide power thereto to produce a dipole magnetic field.

Receiver 36 of a preferred embodiment of the present invention involvesa single antenna location employing two orthogonally disposed antennae.If two orthogonal antennae are used to measure the horizontal andvertical components of a dipole magnetic field, and those components arevectorially added, the magnetic field strength varies as shown in FIG.1b. The orthogonal pair of receiving antennae provide the total magneticfield strength in the plane of the orthogonal antennae axes. As aresult, the indicated distance to transmitter 10 will be a monotonicfunction of the true distance to transmitter 10 along dipole flux lines.Once the location directly above transmitter 10 is determined, a depthreading taken at that position will indicate the true depth, because thefield strength perpendicular to transmitter 10 is approximately zero atthat point.

The surface location directly above transmitter 10 can be found bysearching for the minimum distance reading on the receiver 36 display.Since the monotonic function exhibits only one peak, maximum point 12,locator/monitor operator 26 cannot be misled with respect to transmitter10 location. Two orthogonal antennae in close spatial relationship areuseful in the practice of the present invention, because locator/monitoroperator 26 almost always knows the general direction of the bore. Ifthis direction were also unknown, a third antenna, orthogonal to theplane defined by the other two, could be incorporated in receiver 36. Inthe three-antennae embodiment of receiver 36 of the present invention,the three dimensional components of the magnetic field strength arevectorially added to eliminate all ambiguity regarding transmitter 10position (i.e., true depth or range is indicated at all times, becausethe total field is being measured).

Preferably, the two antennae functioning as depth receivers in thepresent invention are oriented such that one is disposed at a 45° angleto a horizontal plane passing through receiver 36 parallel to the basethereof, and the other is oriented orthogonally thereto. When theantennae are oriented in this configuration and are balanced, andreceiver 36 is located directly over transmitter 10, the induced signal(i.e., the amplified receiving antenna output signal) will be the samefor both antennae. Antennae are not always balanced, however. If theantennae are not balanced, inaccurate transmitter 10 location willresult.

As a result, the present invention may include an automatic antennaebalancing means that may be initiated in or as a prerequisite toreceiver 36 modes, such as antennae balancing, transmitter locating orcalibration, for example. Specifically, an automatic adjustment may bemade to amplified antenna output signal gain of a first antenna tobalance it with a second antenna amplified output. If the output signalfrom a first antenna (OS₁) is not equal to the output signal from asecond antenna (OS₂) at a location where it should be equal, OS₁ will beadjusted by a factor of OS₂ /O5₁ for each subsequent measurement.Consequently, the amplified output signals from the two antennae (i.e.,the antenna/amplifier systems) will be balanced.

Antennae balancing may be accomplished at any point relative totransmitter 10 that equal signal strength is expected at each antennae.Receiver 36 antennae balancing may be conducted, for example, at a pointdirectly over transmitter 10, directly behind or in front of transmitter10 long the longitudinal axis thereof, or the like.

If a spatially-separated, prior art two antennae device is used with adipole field, the sensitivity of that device (i.e., the percentagechange in output signal divided by the percentage change in distance)depends on the ratio of antennae separation to depth. Measurementsensitivity therefore decreases with increasing depth. On the otherhand, if signal strength alone is used in computation as contemplated bythe present invention, the locator/monitor measurement sensitivitybecomes depth-independent. As a result, a locator/monitor operating onsignal strength in an antennae separation-insensitive manner, such asthat of the present invention, could be as much as an order of magnitudemore sensitive at a depth of 10 feet. To achieve depth-independentmeasurement sensitivity, the proportionality constant relating distanceto the cube root of magnetic field strength must be convenientlydeterminable, however.

The aforementioned factors and parameter relationships indicate, and theprior art has recognized, that the distance between transmitter 10 andreceiver 36 can be obtained using magnetic field strength measurementsalone once the proportionality constant has been determined.

The proportionality constant depends upon a variety of parameters, suchas soil characteristics, tool body attenuation and battery strength. Asa result, locator/monitors of the present invention should be calibrated(i.e., the proportionality constant should be determined) before useunder new conditions or after a substantial period of continual use.Since calibration is required often, a simple procedure therefor, asprovided by the present invention, is desirable.

A method to accurately determine the proportionality constant in anantennae separation-insensitive manner is to measure the magnetic fieldstrength at two positions using a "single antenna location" device(e.g., two orthogonal antennae disposed in close spatial proximity),such as the locator/monitor of the present invention. In such a device,the single antenna location is moved between two measurement positionsby an operator. Consequently, the spacing between the two measurementpositions can be much larger than that of a spatially separatedtwo-antennae device, since packaging requirements do not limit thedistance between measurement positions in locator/monitors of thepresent invention.

In the practice of the present invention, the magnetic field strength(B₁) is measured by the orthogonally disposed antennae at a firstposition that is located a distance d₁ from transmitter 10. Similarly,magnetic field strength (B₂) is measured at a second position that isvertically displaced from the first position and located a distance d₂from transmitter 10. If the distance d between the first and secondpositions is known, the variables k, d₁ and d₂ may be calculated bysolving the following equations:

    B.sub.1 =k/d.sub.1.sup.3

    B.sub.2 =k/d.sub.2.sup.3

    d=(d.sub.2 -d.sub.1)

An important feature of this process is that d is accuratelyascertainable. As a result, an accurate independent measurement systemis incorporated into receiver 36 of locator/monitors of the presentinvention, so that the distance between the two measurement positionscan be determined. The independent distance measuring means could alsobe separate from the receiver, but such a configuration is notpreferred.

One method of achieving such accurate measurement is the use anultrasonic measuring device to precisely reference the elevation ofreceiver 36 above surface 32. An ultrasonic system measures distance bymonitoring the time it takes a signal to travel from an ultrasonictransmitter to the surface and back to an ultrasonic receiver. Atemperature sensor is preferably included in the ultrasonic measuringdevice to measure the ambient temperature and correct for the speed ofsound variation with temperature. knowing the distance between themeasurement locations d and the two magnetic field strengths B₁ and B₂,the proportionality constant k and transmitter 10 depth can bedetermined.

FIG. 5a shows a block diagram of a preferred embodiment of a receiver 36useful in the present invention. Receiver 36 includes three separatereceiving units: a first range receiver 122, a second range receiver 124and a roll/pitch receiver 126. First and second range receivers 122 and124 preferably involve antennae arranged orthogonally with respect toeach other, measuring the vertical and horizontal components,respectively, of the magnetic field emanating from transmitter 10. Rangereceivers 122 and 124 have very narrow band-pass filters preferablycentered on the carrier frequency that strip the modulation side-bandsfrom received signal to provide a steady amplitude carrier signal usedfor range computation by a CPU 128. Roll/pitch receiver 126 demodulatesthe received signal and decodes it into 4-bit nibbles that provide rolland pitch orientation information.

The roll-pitch data and the range signals are fed into a CPU interface130 that converts the analog signals into digital format for processingby CPU 128. CPU interface 130 also sets the gain in range receivers 122and 124 to maintain the signals in the dynamic range of an A/D converterwithin CPU interface 130. CPU interface 130 also accepts signals fromswitches 132 that control receiver 36 functions. Other functions of CPUinterface 130 are to drive a display system 134, a signal beeper 136 andan ultrasonic ranging system 138, such as an ultrasonic transducer.

Antennae range receivers 122 and 124 used in receiver 36 of the presentinvention differ from the spatially separated antennae used in prior artdevices. Antennae range receivers 122 and 124 measure differentcomponents of the magnetic field emanating from transmitter 10 and arelocated in spatial proximity to each other.

The block diagram of an alternative and preferred embodiment of receiver36 is shown in FIG. 5b. In this embodiment of the present invention,roll-pitch receiver 126 includes a tuned antennae system 150 composed ofa coil 152 and a variable capacitor 154. Receiver 126 communicates withCPU 128 through data bus 156 and data strobes 158. Each range receiver122,124 has an associated linear antennae 160 and 162, respectively,which are orthogonally disposed. The axes of antennae 160 and 162 may,for example, be offset 45° from a horizontal plane passing throughreceiver 36 parallel to the base thereof. Gain control buses 164 and 166permit CPU 128 to set the gain values of range receivers 122 and 124.Range receivers 122 and 124 produce an output voltage related to therange from receiver 36 to transmitter 10 and the controlled gain settingof CPU 128. These voltages are fed to a multiplexer andanalog-to-digital (A/D) converter 168 through a set of wires 170 and172. Multiplexer-A/D converter 168 is controlled by a control bus 174from CPU 128. Channel selection is performed by CPU 128 through controlbus 174, and the digitized data are returned to CPU 128 by means of adata bus 176.

A temperature sensor 180 and associated electronics 178 form a part ofultrasonic ranging system 181, including an ultrasonic transmitter 186,an ultrasonic receiver 188 and associated electronics 182 and 184,respectively. Ultrasonic transmitter 186 generates an ultrasonic pulseof sufficient strength and duration to facilitate accurate ultrasonicreceiver 188-to-surface 32 measurements. Such pulses may, for example,range from about 30 kHz to about 60 kHz and extend from about 0.25 ms toabout 5 ms. A transmitted ultrasonic pulse of approximately 40 kffz and1 ms duration, for example, is initiated by CPU 128 through a strobeline 190. CPU 128 measures the time between pulse transmission and pulsereturn, communicated to CPU 128 through a line 192. CPU 128 thencalculates the receiver 36-surface 32 range based on the time andambient temperature.

A control switch 194 provides operator input signals to CPU 128 tocontrol power switching and the various operational modes (e.g.,calibration, location, depth measurement, peak signal holding, and rangecompensation).

Beeper 136 provides operational mode information as well as confirmationand error signaling. Beeper 136 may also be activated during transmitter10 locating processes as described herein. Data are presented on display134. Display 134 is preferably configured to supply information onlocation and orientation of transmitter 10 as well as receiver 36battery status. Other useful data may also be displayed, if desired.

Receiver 36 is capable of constantly comparing rates of change (i.e.,gradients) of the vectorially added magnetic field strength componentsto provide locator/monitor operator 26 with an indication of hisdirection of motion relative to transmitter 10 (i.e. toward or away fromtransmitter 10). Once positioned substantially directly abovetransmitter 10, operator 26 can rotate receiver 36 to the left or rightto determine the yaw orientation of boring apparatus 24 using thedisplayed field strength rate of change. This operator 26 position isalso appropriate for obtaining accurate depth measurements.

The individual components of receiver 36 are known and commerciallyavailable. For example, ultrasonic measurement devices useful inreceiver 36 are ME 251-1603 (Mouser) and P9934-ND and P9935-ND(Panasonic). As a result, one of ordinary skill in the art couldconstruct and implement receivers 36 as contemplated by the presentinvention.

Optical means, such as used for camera focusing, or mechanical means mayalternatively be employed to determine the distance between magneticfield strength measurement positions. One of ordinary skill in the artwould be able to design and implement these distance measuring means.

Since receiver 36 of the locator/monitor of the present invention hasonly one antennae location, it can be very compact. The prior art twoantennae location systems, for example, must accommodate the antennaeand the fixed separation therebetween.

The locator/monitor of the present invention is self-calibrating, inthat the proportionality constant between magnetic field strength andthe inverse cube of the distance between transmitter 10 and receiver 36can be recomputed at any time. Recalibration might be undertaken whenconcern about transmitter 10 output or ground attenuation deviation,receiver antennae 122 or 124 alterations resulting from thermal effects,for example, or when any other concern regarding measurement accuracyarises. The calibration procedure uses the transmitter signal from theboring tool, so that the calibration can be conducted during boring.That is, the calibration process of the present invention is not sodistinct from normal operation as to require a cessation of normaloperation therefor. Since the locator/monitor of the present inventioninvolves only one antenna location, there is only one antennalocation/electronics path. As a result, only the linearity of theresponse in receiver 36 electronics affects measurement accuracy.Fortunately, with modern electronic circuits, linearity is generally nota problem.

Calibration of receiver 36 is performed by holding it close to surface32 and switching it into calibration mode as described herein. Receiver36 measures field strength and the ultrasonic range to surface 32.Receiver 36 is raised a vertical distance above the first measurementlocation, and a second set of measurements is recorded. CPU 128 combinesthe data from the measurement sets; calculates the range proportionalityconstant; and stores the information. Notification of correctcalibration procedures are accomplished by display 134 and beeper 136.

Digital processing allows for verification of signals by comparingreadings and other tests as described herein. Also, CPU 128 circuitrycan compensate for the height that operator 26 holds receiver 36 abovesurface 32. This feature is important when surface 32 obstructions, suchas rocks or landscaping, are located at a measurement position. Such anobstruction falsely alters the level of surface 32, thereby falselyincreasing or decreasing (if the obstruction is a ditch or hole of somesort) the depth of transmitter 10. In this situation, operator 26 willemploy ultrasonic ranging system 138 of the locator/monitor of thepresent invention at a location adjacent to but free from theobstruction and at a height greater than that of the obstruction. Thisdistance measurement is stored in CPU 128 memory. Operator 26 deploysreceiver 36 over the obstruction at substantially the same height thatit was deployed at the closely adjacent location. CPU 128 uses thestored distance-to-surface value and displays the depth of transmitter10 below reference or extended surface 32, despite the obstruction.

A preferred pitch sensor useful in the practice of the present inventionis durable and cost effective. Components used to produce a prototypedevice were obtained from plumbing supply, hardware, or hobby stores andconstituted standard fittings and tubing. When used as a pitch sensorfor horizontal boring applications, the sensor of the present inventionis insensitive to roll orientation.

As shown in FIG. 6, a pitch sensor 200 consists of two insulating endcaps 202 and 204, two outer conductive lengths of tubing 206 and 208, aninsulating center coupling 210, a conductive central rod or tube 212 anda conductive fluid 214. Conductive fluid 214 provides a current pathbetween central rod 212 and outer tubes 206 and 208. Tubes 206 and 208are prevented from electrically contacting each other by a gap or ring216 in center coupling 210. As pitch sensor 200 orients to mimic theorientation of tool head 28, conductive fluid 214 flows to one end ofsensor 200 or the other. A larger conductive path will exist betweencentral rod 212 and whichever outer tube 206 or 208 holds the greatervolume of fluid 214. By comparing the conductivities between central rod212 and outer tubes 206 and 208, the pitch angle of pitch sensor 200 canbe determined.

A prototype pitch sensor 200 was constructed from 1/2-inch cpvc plasticwater pipe fittings available from plumbing supply and hardware stores.End caps 202 and 204 were drilled in a lathe to accept central rod 212,a 3/16-inch brass tube purchased from a hobby shop. Outer tubes 206 and208 were short lengths of standard copper water tubing. Conductive fluid214 was glycerin, with a small quantity of saline solution added toprovide for conductivity. Glycerin exhibits a low freezing point and theviscosity necessary for sufficient damping. Prototype pitch sensor 200was cemented together to prevent leakage of fluid 214. The use ofreadily available household parts and simple machining allowed prototypepitch sensor 200 to be manufactured at low cost. One of ordinary skillin the art would be able to construct pitch sensor 200 of the presentinvention.

FIG. 7 shows an electronic circuit 220 capable of driving pitch sensor200 and providing a digital output. An analog output can also be derivedfrom circuit 220 by eliminating an A/D converter 222. Circuit 220consists of an oscillator 224 producing an alternating voltage output.Oscillator 224 may produce any convenient alternating voltage output.Outputs ranging from about 50 Hz to about 10 kHz are appropriate for usein the practice of the present invention. For the prototype, the outputwas a 2 kHz square wave. The output is a.c. coupled to sensor 200through a capacitor 226. Sensor 200 is preferably driven without anyd.c. component to prevent dissociation of conductive fluid 214.Oscillator 224 output is rectified, filtered and scaled by device 228.Output from device 228 is used as a reference voltage 230 for A/Dconverter 222 to compensate for any changes in oscillator 224 outputlevel.

One outer tube 206 or 208 is coupled to capacitor 226. The other outertube 208 or 206, respectively, is connected to an analog ground 232,provided by an operational amplifier 234. Analog ground 232 voltagelevel is sufficiently high that the a.c. peaks remain within theoperational range of circuit 220. This voltage level is determined by aresistor pair 236 and 238.

Pitch sensor output 240 is taken from central rod 212 at an electricalconnection 242. Output signal 240 amplitude is related to the pitchangle of tool head 28. The exact relationship therebetween is determinedby pitch sensor 200 geometry as discussed herein. Output signal 240 isfed into a peak detector 244, including an operational amplifier 246, adiode rectifier 248 and a capacitor 250. A peak detected signal 252,analog ground 232 and reference signal 230 are fed into A/D converter222 that converts the signals to a digital output 254. Since outputsignal 254 is referenced to oscillator 224 output voltage, any changesin output signal 254 due to fluctuations in oscillator 224 outputvoltage will be cancelled.

All of the components of electronic circuits 220 capable of drivingpitch sensor 200 are known and commercially available. As a consequence,a practitioner in the art could implement pitch sensor 200 of thepresent invention.

FIG. 8 shows characteristic output signal 254 curves. For applicationswhere high accuracy over a limited range of pitch is desired, a curve260 would be preferred. For applications where a broad range of pitch isdesired, a curve 262 would be preferred. The variation between curves260 and 262 is controlled by pitch sensor 200 geometry. Pitch sensors200 of the present invention may be sized and configured to produce anoutput signal 254 over the full range of +90 to -90 degrees, if required(e.g., curve 262). Pitch sensors 200 may also be designed to produce itsfull output signal 254 over an extremely small range (e.g., curve 260).

The alterable geometric parameters are the length-to-diameter ratio ofouter tubes 206 and 208, the diameter ratio of central rod 212-to-outertubes 206 and 208 and the relative level of conductive fluid 214 inpitch sensor 200. A very narrow, highly sensitive pitch resolution maybe achieved by broadening the separation between outer tubes 206 and 208and constructing pitch sensor 200 with a high outer tube 206 and 208length-to-diameter ratio.

Other pitch sensor 200 characteristics could be achieved throughstructural alternatives thereof. For example, increasing the amount offluid 214 in sensor 200 may be undertaken to limit the voltage range.

A well-damped output signal 254 can be obtained using a viscous fluid214 in sensor 200. Glycerin or a polymer exhibiting the desired elevatedviscosity may be used for this purpose. If a nonconductive liquid isused to provide the viscosity, a conductive liquid or a salt must beadded to form conductive fluid 214. The required degree of fluid 214conductivity required depends on the associated electronic circuitry220. Since output signal 254 is based on a ratio of conductive paths,pitch sensor 200 performance is insensitive to fluid 214 conductivity.As the conductivity increases, the drive current from oscillator 224 forcircuit 220 shown in FIG. 7 will increase, however. If low power isdesired, then fluid 214 should exhibit low conductivity.

Sensor 200 can be used as an accelerometer, since an acceleration alongthe axis of central rod 212 will cause fluid 214 displacement in thesame manner as a pitch rotation. In an accelerometer application, signaldamping assumes greater significance. As a consequence, viscosity ofconductive fluid 214 must be carefully adjusted for this application.Baffles, porous foam or other known damping devices may be employed toobtain proper fluid 214 characteristics. Alternatively, fluid 214 may bereplaced with conductive balls or other flowing conductive materialcapable of acceleration-induced displacement in the manner of conductivefluid 214.

Pitch sensor 200 may be plated with gold or another appropriate materialto prevent corrosion or reaction between conductive fluid 214 and theinternal surfaces of pitch sensor 200. Such plating would greatly extendthe life of sensor 200 and provide stability to conductive fluid 214 bypreventing additional conductive ions from going into solution.

As demonstrated above, the geometry of and the geometric relationshipsbetween pitch sensor 200 components dictate the performancecharacteristics of pitch sensor 200 of the present invention. Apractitioner in the art would therefore be able to design and implementan appropriate pitch sensor 200 for the particular application in whichit is to be used.

Although the discussion above is directed to electrically conductivefluid 214, a dielectric fluid or other flowing dielectric medium mayalso be employed in pitch sensors 200 of the present invention. In thedielectric fluid embodiment of pitch sensor 200, a voltage output isderived from a comparison of the capacitance between outer tubes 206 and208 and rod 212. A dielectric fluid useful in the present invention is,for example, glycerin, petroleum oils and synthetic fluids. Inputimpedance should be high, greater than about 10 mega-ohms, for pitchsensors 200 employing dielectric fluids. Since the geometricconfiguration of pitch sensor 200 components impacts dielectric fluidsensors 200 in substantially the same manner as conductive fluid 214sensors 200, and the nature and properties of dielectric fluids areknown, a practitioner in the art would also be able to design andimplement appropriate dielectric pitch sensors 200.

In another device which operates on the same principle, the innerconductive member is not located within the first and second outerconductive members, but is instead located between them. For example,the inner member can be a cylinder of approximately the same dimensionsas the outer members and be joined to the outer members by appropriatecoupling means. In use, fluid flows from one of the outer members to theother via the inner member, thereby altering the conductance orresistance between the inner member and each of the outer members. Theseelectrical properties can be measured in much the same manner asdescribed earlier.

The operational characteristics of this device can be adjusted byvarying the dimensions of the three members, in particular thelength/diameter ratios and the separation between the conductivemembers. Also, the sensitivity of the device can me adjusted by varyingthe amount of fluid which is placed within the device.

FIG. 9 shows a configuration of magnetic transmitting antenna 46contained in a conductive housing 270. A magnetic field 272 derived fromantenna 46 induces a voltage in housing 270 that causes electricalcurrent 274 to flow. Current 274 is depicted, for simplicity, as asingle line in FIG. 9, but, in actuality, it is a distribution on thesurface of housing 270 ascertainable from Maxwell's equations. Surfacecurrent 274 induces a counter magnetic field that tends to cancel field272 derived from antenna 46. As the conductivity of housing 270decreases, the intensity of current 274 decreases, thereby diminishingthe countering field intensity. If a perfect insulating housing 270 wereused, no current 274 would flow, and the entire magnetic field 272induced by antenna 46 would radiate unattenuated. Unfortunately, aconductive metal such as steel produces an amount of current 274sufficient to virtually eliminate radiated magnetic field 272.

FIG. 10 shows a means of reducing surface current 274 in conductivehousing 270 to substantially increase radiated magnetic field 272. Theprincipal difference between housing 270' (FIG. 10) and housing 270(FIG. 9) is that one or more narrow elongated slots 280 are formed inhousing 270'. Slots 280 increase current path length which, becausehousing 270' is not a perfect conductor, will increase apparentresistance. This increase in resistance, in turn, reduces surfacecurrent 274 and the resulting, opposing magnetic field. Alternatively,slots 280 may be replaced by a plurality of elongated apertures of anyconfiguration sufficient to increase current path length.

Slot(s) 280 need only be wide enough to disrupt current 274 flow. Suchnarrow gap(s) do not readily allow debris penetration and are easilyfilled to prevent water intrusion. The filler material need only bestrong enough to withstand the torque loads on housing 270', or,alternatively, elastic enough to deform in response to those loads andrestore to its original shape once the loads have been removed. Sinceslot(s) 280 are narrow, the filler material is also substantiallyprotected from abrasion caused by the rock and soil material beingbored. Composite or ceramic materials could therefore be used as fillermaterials to restore most of the torsional rigidity and strength tohousing 270'.

As stated previously, antennae 46 useful in transmitter 10 of thepresent invention are known and commercially available. Similarly,slotted housing 270' of the present invention may be fabricated fromcommercially available steel tubing using known techniques. As a result,a practitioner in the art would be capable of producing and implementingslotted housing 270' of the present invention.

Housing 270 and 270' tests were conducted using steel tubes with insidediameters of approximately 1 inch and a 1.25 inch outside diameter. Anantenna consisting of a ferrite rod, approximately 0.29 inches indiameter by 1 inch long with windings having an outside diameter ofabout 0.45 inches, was centrally placed in each tube. Four tubes weretested having zero slots (housing 270), one slot (housing 270'), twoslots (housing 270') and four slots (housing 270'). Slots were 0.125inches wide by 4.5 to 5.5 inches long. Data were taken using a precisionreceiver of the present invention located 85 inches from the antenna.Table 1 shows the results of this testing. All signal strengths arereferenced to an antenna not contained in a housing, representing 100%of the signal generated by the antenna.

                  TABLE 1                                                         ______________________________________                                        Signal                                                                        Configuration           Signal                                                Ratio      Orientation  Strength Strength                                     ______________________________________                                        No cover   --           4.920    0.0    dB                                    dB                                                                            No slots   --           0.000    -inf.  dB                                    1 Slot     1            1.139    -12.7  dB                                               2            1.098    -13.0  dB                                               3            1.060    -13.0  dB                                               4            1.103    -13.0  dB                                    2 Slots    1            1.945    -8.1   dB                                               2            1.940    -8.1   dB                                    4 Slots    1            2.835    -4.8   dB                                               5            2.835    -4.8   dB                                    ______________________________________                                    

Orientation Notation:

1. Slot toward receiver

2. Slot perpendicular to receiver (90°)

3. Slot away from receiver

4. Slot perpendicular to receiver (-90°)

5. Slot 45° to receiver

The results indicate that the antenna disposed within the housingwithout slots (housing 270) generated no measurable signal at thereceiver. One slot (housing 270') allowed about 22% of the signal topenetrate the housing, with about 0.6 dB variation in signal strengthdependent upon orientation. With 2 slots (housing 270'), virtually novariation in the signal strength with orientation was observable, andbetter than 39% of the signal penetrated the housing. With 4 slots(housing 270'), no variation in signal strength was observed, and morethan 57% of the signal penetrated the housing.

Tests with other antennae were conducted to explore otherstructural/functional relationships involving housing 270' andcomponents disposed therein. If the antenna diameter was increased untilit became a large fraction of the inside diameter of housing 270'(approximately 80%), a noticeable loss of Q (i.e., the ratio of storedenergy to dissipated energy) in the antenna and a substantial decreasein radiated magnetic field were observed. As a result, the housingdiameter is preferably large in comparison to the antenna diameter.

An antenna having a core that was 0.2 inches in diameter by 0.72 incheslong and an outer winding diameter of 0.40 inches showed almostidentical results for signal strength ratios. This indicates that slotlength-to-antenna length ratio is not critical beyond a minimum ratio.The ratio of housing 270' inside diameter-to-antenna diameter does notappear to be critical below a maximum ratio. Preferred embodiments ofthe present invention employ a housing 270' inside diameter-to-antennadiameter ratio of approximately 2.5 or more. Once these ratios areachieved, the number of slots and the length thereof assumes greaterimportance. An increase in slot number produces narrower conductorpaths, increasing the effective resistance and therefore reducing signallosses. The length of the slots increase the conductive path length andtherefore increase the resistance. The number of slots in the cover willbe restricted by structural considerations and will vary from one tooldesign to another. The minimum slot length corresponds to approximately1.5 antenna lengths.

A preferred receiver 36 of the present invention is shown in FIG. 11.Receiver 36 is preferably a convenient size for portable use. Preferredreceiver 36 may, for example, be approximately 1 foot square (X and Ydimensions) by approximately 5 inches wide (Z dimension). A case 290 isformed from a non-magnetic, non-conductive material such as fiberglassor a styrene plastic, such as ABS, so that the magnetic field generatedby transmitter 10 is able to penetrate case 290 without attenuation. Ahandle 292 is provided to allow operator 26 to hold and manipulatereceiver 36. A trigger switch 294 provides all control functions for theoperation of receiver 36 including power switching, calibration, rangecompensation and locating functions. Specifically, trigger switch 294has a variety of settings and/or may be depressed for certain timeperiods or at specific times during use to initiate specific receiverfunctions or as a prerequisite to such functions. Displays 296,298 and300 provide information to operator 26 regarding range to transmitter10, transmitter 10 orientation and receiver 36 battery condition. Beeper136 provides audible cues to facilitate calibration and locatingfunctions as well as to identify errors in procedures or functions.Cover plate 302 provides access to receiver 36 electronics.

A cross-sectional view of receiver 36 is shown in FIG. 12. Displays 296,298 and 300 and associated electronic drives are arranged on a printedcircuit card 304. An orthogonal set of antennae 122 and 124 are attachedto the side of case 290. Power is supplied by a set of batteries (notshown) loaded into an assembly 306 and held in place by battery cover308. An additional set of printed circuit cards 310 are arranged abovean electronics assembly support 312. Attached to electronics assemblysupport 312 is rollpitch receiving antenna 126. Two ultrasonictransducers 314 (i.e., ultrasonic transmitter 186 and ultrasonicreceiver 188) are mounted to the bottom of case 290. Temperature sensor178 is also mounted to the bottom of case 290.

Equipment cases having handles, triggers, displays, battery-containingassemblies and the like are known in the art. Such equipment may befabricated from known materials and components to achieve a portableapparatus. In addition, the electronic/mechanical interface at triggerswitch 294 and electronics relating to displays 296, 298 and 300 arewithin the purview of a practitioner in the art. As a result, one ofordinary skill in the art could design and implement preferred cases 290of the present invention.

In operation, the transmitter is installed in the head of a boring toolused to drill a predominately horizontal hole. The transmitter providesan amplitude modulated signal that consists of the carrier frequency(e.g., 32768 Hz) and two tones that are changed four times over aone-second interval followed by a single tone lasting one second used asa delimiter. The tones preferably range in frequency from about 400 toabout 900 Hz, for example, so that the entire signal is contained in abandwidth of less than 2 kHz, for example. Roll-pitch electronics in thereceiver of the present invention has a bandwidth of less than 2 kHz,for example, to receive the transmitter signal. The depth receivers havea bandwidth of about 20 Hz, for example, so that roll/pitch modulationdoes not influence the depth output signal. The depth output signal is afunction only of the strength of the received carrier frequency.

The inductor-antenna in the transmitter emits a signal that approximatesa dipole field at distances greater than a few antenna coil lengths. Thesignal strength of a dipole field is known to vary as the inverse cubeof the distance. This relationship is used to measure depth andcalibrate the system. Since the transmitter has a well regulated powersupply and stable components, the transmitted signal remains constantwith time so that frequent recalibration is not required.

When necessary or desired, calibration is achieved by first holding thereceiver unit near the surface above the transmitter and depressing theswitch for approximately 2 seconds, for example. The CPU in the receivermeasures the magnetic signal strength of the carrier and uses theultrasonic system to measure the distance to the surface. The receiveris then raised and the switch again briefly depressed. A second set ofmagnetic and acoustic measurements are taken. The relationship betweenthe signal strength and the range is then computed by the CPU and theproportionality constant stored. The distance between the receiver andtransmitter will be shown on the receiver display in one inchincrements. If the operator were to again briefly depress the switch,the receiver would measure the distance to the surface using theultrasonic ranging system and subtract this value from the magneticrange to obtain the distance of the transmitter below the surface.

An expedited transmitter location process is also provided by thepresent invention. This process takes advantage of the fact that theorthogonal antenna system measures the total magnetic field strength inthe plane of the antennae. When the receiver case (antenna plane) ispointed in a direction parallel to one of the dipole flux linesemanating from the transmitter, the measured field will be a maximum.The operator can therefore hold the receiver at a fixed location abovethe surface and rotate the case until a minimum distance to thetransmitter is indicated. The operator can then move in the indicateddirection for a distance related to the indicated distance and repeatthe process. Repetition continues until the operator passes over thetransmitter, as indicated by an increase in range. The location processmust be accomplished in stages, because flux lines are not, in general,straight lines to the transmitter.

The sensitivity of the expedited locating process can be increased byusing the square of the signal strength rather than the range which hasan inverse cube root relation to the signal strength. Since the squareof the signal strength is available from the vector sum process used toobtain the total in-plane signal strength, its use does not addsignificantly to the computational process. By comparing the currentsignal strength with the previously measured one, a sensitive signalpeak can be determined.

The process consists of arming a beeper activation circuit after severalconsecutive signal strength increases are measured, indicating a peaksearching mode. When the signal ceases to increase or decreases, a briefactivation of the beeper occurs. With a measurement cycle time of 0.1 ofa second, for example, the indication is quite accurate for moderaterates of signal strength change. In order to enhance the accuracy atfaster rates of change, a predictor method may be used to estimate therate of change of signal strength. If the predictor method determinesthe peak will occur before the next measurement, the routine measurementcycle is halted, and the beeper is activated after a delay estimated tobe that required to reach peak signal.

The predictor may, for example, fit a quadratic function to three priormagnetic field strength readings to determine whether the field strengthwill pass through a maximum (i.e., zero slope point) prior to the nextreading. If the predictor determines that a maximum will occur prior tothe next reading, it suspends the next measurement cycle; waits untilthe estimated time to the maximum passes; and initiates the beeper. Thepredictor of the present invention is also preferably capable ofascertaining circumstances when extrapolation will not be accurate.Under these conditions, the predictor will not suspend the measurementcycle. If the predictor does not suspend the measurement cycle, and thesubsequent measurement is less than the prior measurement, the predictorwill initiate the beeper.

A case rotation process is used to determine the direction that thetransmitter is pointed once the location of the transmitter has beenfound. The case rotation process may operate in the same manner as thelocator process described above. Specifically, a predictor-controlledextrapolation process may be employed.

The peak signal squared value derived from the searching process may beheld in memory. This peak held signal corresponds to the last measuredmagnetic field strength reading, rather than the extrapolated value. Thereceiver of the present invention will hold the signal for a specifiedtime period, for example, 2 seconds, to allow the operator the option tofurther refine the searching process by comparing the current signalstrength to the peak value. This is accomplished by the operator, forexample, by depressing the switch within the time period for holding thepeak signal after the peak signal beep sounds. As long as the switchremains depressed, the receiver will compare the current signal with thepeak value and activate the beeper if the current signal strength equalsor surpasses the peak held value.

This feature is useful for accurately locating the transmitter. If theoperator holds the receiver closer to the surface than it was when thepeak beep was heard, there will be an area above the surface where thebeeper will sound. As the receiver is raised the area will becomesmaller. Eventually, a beep will sound at only one location. Thisprocess provides a very accurate location. A similar process can be usedto establish the pointing direction (i.e., yaw) of the transmitter,replacing lateral and longitudinal displacements with rotation.Specifically, an initial wide angle in which the beeper function isactivated will narrow as the receiver is elevated, until the yaworientation is pinpointed.

The signal strength comparison may also be presented visually using a+/- sign on a display, for example. If the signal strength isincreasing, indicating a reduction in range, the - sign is displayed. Ifthe signal strength is decreasing, indicating an increase in range,the + sign is displayed.

The digital signals received by the roll-pitch receiver and sent to theCPU are decoded and displayed. The roll orientation may, for example, berepresented as clock positions in 30 degree increments, 1 through 12.The pitch may, for example, be displayed in degrees from +90 to -90. Anindication may additionally be provided when the roll and pitch displaysare updated by flashing a sign on the pitch display.

A sign may also be used to indicate that the receiver should berecalibrated. This determination is established whenever the temperatureof the receiver has changed at least 10° C. since the last calibration.Any other appropriate criteria may also be used in making therecalibration determination.

A timer in the CPU determines whether the switch has been activated inthe past 5 minutes, for example. If the switch has not been activatedwithin the appropriate time frame, the receiver is switched off.Depressing the switch returns power to the receiver.

The bandwidth values, time constraints, trigger switch activationparticulars and the like presented above are exemplary. Otherappropriate and substantially equivalent indicators or procedures may beused to accomplish these tasks. A practitioner in the art could produceand implement a receiver case housing with appropriate operationalmechanics, electronics and electromechanics.

A procedure for locating the transmitter is based on having the tworeceiving antennae oriented at a 45 degree angle to level. With thisorientation the signal strength in the two antennae will balance atthree locations along a line in a vertical plane containing the axis ofthe transmitting antenna. One location will be approximately above thetransmitting antenna, one will be behind the transmitting antenna andone will be ahead of the transmitting antenna. Referring to FIG. 12, ifthe receiving antennae are moved forward (to the left) along the linestarting from a substantial distance from the transmitting antenna, thesignal strength would be greater in antenna 124 than in antenna 122assuming that the transmitting antenna was to the left of the receivingantennae. As the receiving antennae are moved forward (to the left) thesignal strength will reach a balance point where the flux line from thetransmitting antenna through the receiving antennae is vertical. As thereceiving antennae are moved farther toward the transmitting antenna,the signal strength will be greater in antenna 122 until the point wherethe strengths in both antennae are balanced. At this point, the fluxline is horizontal and the receiving antennae are substantially over thetransmitter. The exact location of the balance will vary slightly due tothe pitch angle of the transmitting antenna. Beyond this mid-balancepoint, the signal in antenna 124 will be greater until a third balancepoint is reached when the flux line through the receiving antennae isagain vertical. Beyond this fore-balance point, the signal strength willagain be greater in antenna 122.

By noting the transition of greater signal strength between antennae 122and 124 at the balance point, it is possible to distinguish the pointsubstantially over the transmitting antenna from the other two points ofbalance. That is, there is only one transition of greater signalstrength from antenna 122 to antenna 124 while there are two transitionsfrom 124 to 122.

The two locations where greater signal strength transitions from antenna124 to antenna 122 can be used to provide two lateral locates which arein planes perpendicular to the axis of the transmitting antenna, oneahead and one behind. This locating procedure indicates the lateralposition and direction of the transmitter. At these two balance points,the magnetic flux lines from the transmitting antenna extend in a radialdirection along a circle passing through the receiving antennaecontained in the planes which are substantially perpendicular to theaxis of the transmitter. If the receiving antennae are rotated about avertical axis such that the receiving antennae axes are in the sameplane as the radial flux line, then another signal strength balancepoint can be found. This point will be on the line formed by theintersection of the vertical plane through the transmitting antenna andthe plane of the radial flux lines. A mark or flag can be placed on thesurface of the ground directly below the point. A corresponding pointcan also be found on the other end of the transmitting antenna and thesurface of the ground marked accordingly. The line connecting the twopoints will be in the vertical plane containing the axis of thetransmitter and therefore provide the lateral locate. Then the balancepoint at the transition of greater signal strength from antenna 122 toantenna 124 as the receiver is moved forward along the line will providea location substantially over the center of the transmitting antenna.

An important feature of the procedure just outlined for the laterallocate is the use of the vertical component of the flux field to obtaina balance when the plane of the receiving antennae is perpendicular tothe axis of the transmitting antenna. This means that any point behindor ahead of the balance point substantially above the transmittingantenna could be used since the flux lines at any other location exceptsubstantially over the transmitting antenna would have a verticalcomponent. Although the best sensitivity will be obtained by using thefore and aft balance points, the procedure will work without having tofind these points.

In practice, acceptable lateral locates have been obtained by firstfinding the mid-balance point above the transmitting antenna, thenmarking the location by having the locating operator place their foot onthe ground below the receiving unit and then pivoting on that foot so asto be able to move the receiver perpendicular to the axis of thetransmitter within normal reach ahead or behind the marked location.

A similar locating process can be accomplished employing only onehorizontal receiving antenna. With the single antenna configuration, thelocations where the flux lines are substantially vertical would cause anull signal in the receiving antenna, and the location substantiallyover the transmitting antenna would cause a maximum or peak in theinduced signal strength in the receiving antenna. One difference betweenthe single-antenna and the dual-antennae locating methods is that withthe single-antenna method there is no indication as to the direction tomove to find the null or maximum such as the relative signal strengthdescribed above.

In practice, a "+" and "-" symbol have been used on the receiver'sdisplay to denote which receiving antenna has the greater induced signalstrength. If antenna 124 has the greater signal strength, a "+" isdisplayed, and if antenna 122 has the greater signal strength, a "-" isdisplayed. Moving the receiver forward toward the transmitting antennafrom a great distance, the display will first show a "+" then switch toa "-" at the aft balance point where the flux line is substantiallyvertical. Moving on in the same direction, the display will thentransition to a "+" at the balance point substantially over thetransmitting antenna where the flux line is level. Moving on farther,the display will finally transition to a "-" at the fore balance pointwhere the flux line is again substantially vertical. by finding "-" to"+" transition, the balance point substantially over the transmittingantenna can be distinguished from the other two balance points whichwill be "+" to "-" transitions.

The complete locating procedure using the fore and aft signal balancepoints have been found to provide very good accuracy not only for thelocation of the transmitting antenna but also for the direction that theantenna is pointing.

A receiver as is described in this application can also be used toidentify and measure the angular and horizontal displacement of atransmitter. When used for this purpose, the orthogonally orientedantennae are in a horizontal plane. The two antennae detect the signalfrom the transmitter which is horizontally displaced from the receiverand at a distance from the receiver. For example, the receiver may belocated at a position to which a boring tool is directed. The boringtool can be started towards the location of the receiver from a locationat a distance from the receiver. As the boring device progresses towardsthe receiver, the receiver can detect when the flux line from thetransmitting antenna through the receiving antennae are deflected so asnot to cause equal signals to be induced. Such a deflection can becaused by an angular deflection, a lateral deflection or a combinationof both.

When the boring device is angularly and/or laterally displaced from adirect flux line path towards the receiver, one of the two orthogonallyoriented antennae will detect a stronger signal than the other antennaand this will be indicative of the direction in which the boring devicemust be steered. An appropriate visual indication can be given to theoperator as to which of the two antennae is receiving the greater signaland thus the direction to which the boring device must be steered. Thisenables the operator to correct the direction of the boring device'sprogress.

Optionally, the CPU can calculate the displacement of the boring toolfrom a flux line heading to the receiver as a function of the ratiobetween the signal strengths measured by the two antennae. Thisinformation can be useful in helping the operator to determine thedegree of correction which is required. It is also possible to reduce oreliminate the need for an operator by providing an appropriateconnection between the receiving unit and the boring control device.When the receiver senses that the boring tool is displaced from a fluxline course to the receiver, it can transmit to the boring controldevice an indication of the direction and, optionally, the magnitude ofthe error. In response to this input, the boring control device canautomatically adjust the direction in which the boring tool moves inorder to bring the tool back into a flux line path towards the receiver.

In yet another version of such a control device, the receiver includestwo pairs of receiving antennae, the antennae in each pair beingorthogonally oriented to each other, with one pair in a horizontal planeand the other pair in a vertical plane. The antenna pair in thehorizontal plane functions to provide an indication of the displacementof the boring tool as described above, and the vertically oriented pairprovides an indication of the vertical displacement of the boring toolin a similar manner. Such a device can provide simple, reliable andautomatic control progress of a boring tool.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for purposes of illustration, it will be apparent tothose skilled in the art that the invention is susceptible to additionalembodiments and that certain of the details described herein may bevaried considerably without departing from the basic principles of theinvention.

What is claimed is:
 1. A locator for continuously locating a magneticfield transmitter disposed beneath a surface, comprising:first andsecond range sensors capable of measuring the field strength of thesignal transmitted by the transmitter and oriented orthogonally to eachother, processor means for generating a vector sum of the fieldstrengths measured by the first and second sensors, display means forindicating the distance between the locator and the transmitter, andcalibration means includingmeans for measuring the distance between thelocator and a reference surface, means for storing a measurement of thefield strengths sensed by the range sensors at least two distances fromthe reference surface and for storing the distances from the referencesurface at which the field strengths were measured, means forcalculating the proportionality constant of field strength of the signalgenerated by the transmitter from the stored measurements of fieldstrength and distance, and wherein the processor means generates amonotonic function indicative of the distance between the locator andthe transmitter on the basis of the calculated proportionality constant.2. A locator according to claim 1, wherein the means for measuring thedistance between the locator and the reference surface is an ultrasonicranging device.
 3. A locator according to claim 1, having aforward/rearward axis, wherein the range sensors each comprise anantenna which is located in a vertical plane and each of which isoriented at an angle of 45° to the forward/rearward axis.
 4. A locatoraccording to claim 3 further comprising means for displaying anindication of which of the first and second range sensors is measuringthe greater field strength.
 5. A method for locating a magnetic fieldtransmitter which is beneath an original point of observation and whichis displaced forwardly or rearwardly from the original point ofobservation, comprising the steps of:measuring the field strength at theoriginal point of observation with a first and a second field strengthmeasuring means, each measuring means comprising an antenna locatedsubstantially within the plane of the transmitter and within which liesthe axis along which the transmitter is forwardly or rearwardlydisplaced, with the first antenna oriented at an angle of 45° above thehorizontal in the forward direction and the second antenna oriented atan angle of 45° above the horizontal in the rearward direction, with thetwo antennas at right angles to each other, moving the detection meansalong a detection path, which is within the vertical plane in adirection from the original point of observation which causes the fieldstrength measured by the field strength measuring means to increase,comparing the field strengths measured by each of the field strengthmeasuring means while moving the detection means along the detectionpath, identifying a first point along the detection path at which theflux lines emitted by the transmitter are substantially vertical, asindicated by the measurement of equal field strengths by the fieldstrength measuring means, identifying a second point along the detectionpath at which the flux lines emitted by the transmitter aresubstantially vertical, as indicated by the measurement of equal fieldstrengths by the field strength measuring means, identifying a pointbetween the first and second points at which point the flux linesemitted by the transmitter are substantially horizontal, as indicated bythe measurement of equal field strengths by the field strength measuringmeans, this point being approximately directly above the transmitter. 6.A locator for continuously locating a dipole magnetic field transmitterdisposed beneath a generally horizontal surface, comprising:a mainhousing defining a forward/rearward axis which extends in a horizontaldirection during normal operation of the locator device; first andsecond antennae capable of measuring the field strength of the signaltransmitted by the transmitter and oriented orthogonally to each other,each of said antennae being carried by said housing in a vertical planeand at an angle of 45° to said forward/rearward axis during normaloperation of the locator device; processor means contained within saidhousing for generating a vector sum of the field strengths measured bythe first and second antennae; and display means contained within thehousing for indicating the distance between the locator and thetransmitter.
 7. A locator according to claim 6 further comprising meansfor displaying an indication of which of the first and second antennaeis measuring the greater field strength.
 8. A method for determining thedistance between a point of observation and a generally horizontallyextending dipole magnetic field transmitter which is beneath the pointof observation and disposed within a given vertical plane, comprisingthe steps of:measuring the field strength of the magnetic field at thepoint of observation with two antennae disposed within said givenvertical plane and oriented orthogonally to each other at angles of 45°with the horizontal, calculating the distance between the point ofobservation and the transmitter as a function of the vector sum of thefield strengths measured by the sensors, and displaying the calculateddistance.
 9. A locator for continuously locating a magnetic fieldtransmitter which is disposed beneath a surface and which transmits amagnetic field signal, comprising:first, sensor means for measuring thefield strength of the signal transmitted by the transmitter, second,processor means connected with said first means for electronicallyprocessing said measured field strength, third, display means connectedto said second means and responsive to said processed field strength forindicating the distance between the locator and the transmitter, andcalibration means includingmeans for measuring the distance between thelocator and a reference surface when the locator is at two differentdistances from the reference surface, means for storing a measurement ofthe field strengths measured by the sensor means when the locator is atsaid two distances from the reference surface and for storing thedistances from the reference surface at which the field strengths weremeasured, means for calculating the proportionality constant of fieldstrength of the signal generated by the transmitter from the storedmeasurements of field strength and distance, and wherein the processormeans generates the monotonic function indicative of the distancebetween the locator and the transmitter on the basis of the calculatedproportionality constant.
 10. A method of continuously locating a dipolemagnetic field transmitter disposed beneath a generally horizontalsurface, comprising:measuring the field strength of the signaltransmitted by the transmitter using a locator including a housing whichdefines a forward/rearward axis which is held in a horizontal directionand first and second antennae oriented orthogonally to each other, eachof said antennae being carried by said housing in a vertical plane andat an angle of 45° to said forward/rearward axis, generating a vectorsum of the field strengths measured by the first and second antennae,and indicating the distance between the locator and the transmitter. 11.A method of locating a magnetic field transmitter located beneath theground from a point above the ground, comprising:(a) positioning themagnetic field transmitter within the ground; (b) causing thetransmitter to produce a magnetic field which is independent of roll bythe boring tool about its axis; (c) from above ground, using a locatormeasuring the field strength of said magnetic field at a number ofdifferent points and from said measurements determining the location ofsaid transmitter; and (d) calibrating said locator by(i) measuring thedistance between the locator and a reference surface, (ii) storing ameasurement of the field strengths sensed by the locator at at least twodistances from the reference surface and for storing the distance fromthe reference surface at which the field strengths were measured, (iii)calculating the proportionality constant of field strength of the signalgenerated by the transmitter from the stored measurements of fieldstrength and distance, whereby said locator is able to generate amonotonic function indicative of the distance between the locator andthe transmitter on the basis of the calculated proportionality constant.12. A method of locating an elongated boring tool located beneath theground from a point above ground, comprising:(a) positioning a dipolemagnetic field transmitter within said boring tool such that thetransmitter extends substantially coaxial with the axis of the tool; (b)causing the transmitter to produce a dipole magnetic field which isindependent of roll by the boring tool about its axis; and (c) fromabove ground, using a locator having a pair of orthogonally positionedantennae, measuring the vector sum of the field strength of saidmagnetic field at a number of different points and from saidmeasurements determining the location of said boring tool.
 13. A methodaccording to claim 12 wherein said locating device includes:first andsecond range sensors capable of measuring the field strength of thesignal transmitted by the transmitter and oriented orthogonally to eachother, processor means for generation a vector sum of the fieldstrengths measured by the first and second sensors, and display meansfor indicating the distance between the locator and the transmitter. 14.A method according to claim 12 including calibrating said locator by:(a)measuring the distance between the locator and a reference surface, (b)storing a measurement of the field strengths sensed by the range sensorsat least two distances from the reference surface and for storing thedistance from the reference surface at which the field strengths weremeasured, (c) calculating the proportionality constant of field strengthof the signal generated by the transmitter from the stored measurementsof field strength and distance, and whereby the locator is able togenerate a monotonic function indicative of the distance between thelocator and the transmitter on the basis of the calculatedproportionality constant.
 15. An arrangement for locating an elongatedboring tool located beneath the ground from a point above ground,comprising:(a) a dipole magnetic field transmitter positioned withinsaid boring tool such that the transmitter extends substantially coaxialwith the axis of the tool; (b) means for causing the transmitter toproduce a magnetic field which is independent of roll by the boring toolabout its axis; and (c) a locator device having two orthogonallypositioned antennae positionable above ground for measuring the vectorsum of the field strength of said magnetic field at a number ofdifferent points and from said measurements determining the location ofsaid boring tool.
 16. An arrangement according to claim 15 wherein saidlocating device includes:first and second range sensors capable ofmeasuring the field strength of the signal transmitted by thetransmitter and oriented orthogonally to each other, processor means forgenerating a vector sum of the field strengths measured by the first andsecond sensors, and display means for indicating the distance betweenthe locator and the transmitter.